Planar lighting device

ABSTRACT

The absolute value of the light amount is increased by providing main light sources and auxiliary light sources on four sides of a light guide plate having a flat and rectangular light exit plane whereas the amounts of light emitted by LED chips of the main light sources and the auxiliary light sources are adjusted such that illuminance is highest near the central area of the light guide plate to obtain a high-in-the-middle, bell-curve illuminance distribution for light emitted by the light guide plate. A planar lighting device achieving a reduced thickness and weight and capable of emitting an increased amount of illumination light is provided.

The entire contents of literature cited in this specification areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a planar lighting device comprisinglight sources and a light guide plate for admitting light emitted by thelight sources and emitting the light through the light exit planethereof. The inventive planar lighting device is used for indoor andoutdoor illumination or as a backlight to illuminate the liquid crystaldisplay panel used in liquid crystal display devices or as a backlightused for advertising panels, advertising towers, advertising signs, andthe like.

Liquid crystal display devices use a backlight unit for radiating lightfrom behind the liquid crystal display panel to illuminate the liquidcrystal display panel. A backlight unit is configured using opticalmembers comprising a light guide plate for diffusing light emitted by anillumination light source to irradiate the liquid crystal display panel,a prism sheet, and a diffusion sheet.

Currently, large liquid crystal display televisions predominantly use aso-called direct illumination type backlight unit having no light guideplate but comprising optical members such as a diffusion plate disposedimmediately above the illumination light source. This type of backlightunit comprises cold cathode tubes serving as a light source provided onthe rear side of the liquid crystal display panel whereas the inside ofthe backlight unit provides white reflection surfaces to secure uniformlight amount distribution and a necessary brightness.

To achieve a uniform light amount distribution with the directillumination type backlight unit, however, the backlight unit needs tohave a given thickness, say about 30 mm, in a direction perpendicular tothe liquid crystal display panel. While demands of still thinnerbacklight units are expected to grow in the future, achieving a furtherreduced thickness of say 10 mm or less with a backlight unit isdifficult in view of uneven light amount distribution expected toaccompany the direct illumination type.

Among backlight units that allow reduction of thickness thereof is abacklight unit using a light guide plate whereby light emitted byillumination light sources and admitted into the light guide plate isguided in given directions and emitted through a light exit plane thatis different from the plane through which light is admitted.

There has been proposed a backlight of a type described above using alight guide plate formed by mixing scattering particles for diffusinglight into a transparent resin, for which reference may be had, forexample, to JP 07-36037 A, JP 08-248233 A, JP 08-271739 A, and JP11-153963 A.

JP 07-36037 A, for example, discloses a light diffusion light guidelight source device comprising a light diffusion light guide memberhaving at least one light entrance plane region and at least one lightexit plane region and light source means for admitting light through thelight entrance plane region, the light diffusion light guide memberhaving a region that has a tendency to decrease in thickness with theincreasing distance from the light entrance plane.

JP 08-248233 A discloses a planar light source device comprising a lightdiffusion light guide member, a prism sheet provided on the side of thelight diffusion light guide member closer to a light exit plane, and areflector provided on the rear side of the light diffusion light guidemember. JP08-271739 A discloses a liquid crystal display comprising alight emission direction correcting element formed of sheet opticalmaterials provided with a light entrance plane having a repeatedundulate pattern of prism arrays and a light exit plane given a lightdiffusing property. JP 11-153963 A discloses a light source devicecomprising a light diffusion light guide member having a scatteringpower therein and light supply means for supplying light through an endplane of the light diffusion light guide member.

In the planar lighting devices provided with a light diffusion lightguide plate containing light scatterers mixed therein as disclosed inthe above prior art literature, light emitted by the light source andadmitted through the light entrance plane into the light diffusion lightguide member receives a single or a multiple scattering effect at agiven rate as the light propagates through the inside of the lightdiffusion light guide member. Moreover, a significant proportion oflight that reaches both end planes of the diffusion light guide memberor a surface of the reflector receives reflection effect and is returnedback into the diffusion light guide member.

The above composite process produces light beam that is emitted throughthe light exit plane highly efficiently with a directivity to travelobliquely forward as viewed from the light source. Briefly, lightradiated by the light source is emitted through the light exit plane ofthe light diffusion light guide member.

Thus, the prior art literature mentioned above purportedly states that alight guide plate containing scattering particles mixed therein iscapable of emitting uniform light with a high light emission efficiency.

As regards the light guide plate used in the planar lighting device,there have been disclosed a light guide plate in the form of a flatplate and a light guide plate composed of a portion shaped to have aregion with a tendency to grow thinner with the increasing distance fromthe light entrance plane attached to the other portion, in addition tothe light guide plate mentioned above that is shaped to have a regionwith a tendency to grow thinner with the increasing distance from thelight entrance plane.

SUMMARY OF THE INVENTION

However, to achieve increased dimensions with the planar lighting deviceusing any of the light guide plates disclosed in the above prior artliterature, light needs to travel a longer distance from the lightsource, which in turn requires the light guide plate itself to be madethicker. Thus, an attempt to enlarge a display screen or a display areaof a planar lighting device is confronted with difficulties in reducingthe thickness and the weight of the planar lighting device.

Thus, an object of the present invention is to provide a planar lightingdevice that eliminates problems accompanying the above prior art andallows a greater amount of illumination light to be emitted with athinner design and a lighter weight.

To solve the above problems, the present invention provides a planarlighting device comprising:

a light guide plate including:

-   -   a light exit plane being flat and rectangular, for emitting        planar light;    -   four light entrance planes, each for admitting light traveling        parallel to the light exit plane, and each being formed along        each of four sides of the light exit plane; and    -   a pair of rear planes being formed on a side opposite from the        light exit plane and inclined such that a thickness of the light        guide plate in a direction perpendicular to the light exit plane        grows thicker with an increasing distance from each of a pair of        opposite light entrance planes in the four light entrance planes        and joining each other in a middle between the pair of opposite        light entrance planes; and

four light sources, each for emitting each of the four light entranceplanes and for admitting the emitted light through the four lightentrance planes, and each being disposed opposite each of the four lightentrance planes of the light guide plate,

wherein each of the four light sources has LED chips and a support forsupporting the LED chips, and

wherein the LED chips of each of the four light sources are arrayed on aplane of the support facing each of the four light entrance planes in alongitudinal direction of each of the four light entrance planes.

According to the invention as described above, the LED chips preferablycomprise LEDs each emitting different amounts of light, respectively.

Further, the LED chips are preferably adjusted independently of eachother to their respective amounts of light.

Further, an amount of light I of each of the LED chips is preferablyadjusted to satisfy 0<I≦1 when an amount of light of one of the LEDchips adjacent a central portion of each light entrance plane is 1.

Further, the four light entrance planes preferably comprises:

a first light entrance plane and a second light entrance plane beingformed on a pair of opposite sides of the light exit plane andconstituting the pair of opposite light entrance planes, and

a third light entrance plane and a fourth light entrance plane beingformed on another pair of opposite sides of the light exit plane andconstituting another pair of opposite light entrance planes, wherein thepair of rear planes are composed of a pair of inclined planes that areparallel to the light exit plane in a direction along the first lightentrance plane and the second light entrance plane, respectively, comeclosest to the light exit plane at the first light entrance plane andthe second light entrance plane, respectively, and are distancedfarthest from the light exit plane along a central joint of the pair ofrear planes joining each other in the middle between the first lightentrance plane and the second light entrance plane.

Further, each of the LED chips preferably has a configuration satisfyingan inequality b>a where “a” denotes a length in a thickness direction ofthe light guide plate and “b” denotes a length in a directionperpendicular to the thickness direction of the light guide plate.

Preferably, the light guide plate contains therein numerous scatteringparticles, and satisfies following inequalities:27/100000<(D2−D1)/(L/2)<26/1000 and0.04 Wt %<N_(p)<0.25 Wt %where N_(p) denotes a density of the scattering particles, L denotes adistance between the pair of opposite light entrance planes, D1 denotesa thickness at one of the pair of opposite light entrance planes, and D2denotes a thickness at a center of the light guide plate.

Preferably, the light guide plate contains therein numerous scatteringparticles and satisfies following inequalities:1.1≦Φ·N _(p) ·L _(G) ·K _(C)≦8.20.005≦K_(C)≦0.1where Φ denotes a scattering cross section of the scattering particles,N_(p) denotes a density of the scattering particles, K_(C) denotes acompensation coefficient, and L_(G) denotes a length in an incidentdirection of the light from one of the pair of opposite light entranceplanes to a position where the thickness of the light guide plate in thedirection perpendicular to the light exit plane is thickest.

According to the present invention, the above configuration provides aplanar lighting device permitting increase of the absolute value of thebrightness of light emitted through the light exit plane withoutincreasing the thickness of the light guide plate and capable ofemitting a large amount of illumination light through the light exitplane with a reduced thickness and weight. This permits adapting to adisplay of enlarged dimensions, in particular to a display of enlargeddimensions for a liquid crystal display device such as a liquid crystaltelevision.

Further, according to the invention, light can be emitted through thelight exit plane in a high-in-the-middle, bell-curve brightnessdistribution by adjusting the amount of light of each light source orother factor, providing a planar lighting device that can be suitablyused for a liquid crystal display device such as a liquid crystaltelevision.

Further, the invention provides a planar lighting device capable ofemitting light varying in brightness from area to area as determined onthe light exit plane.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will be apparent from the following detailed description andaccompanying drawings in which:

FIG. 1 is a schematic perspective view illustrating an embodiment of aliquid crystal display device using the planar lighting device of theinvention.

FIG. 2 is a cross sectional view of the liquid crystal display deviceillustrated in FIG. 1 taken along line II-II.

FIG. 3A is a view of an example of the planar lighting deviceillustrated in FIG. 2 taken along line III-III; FIG. 3B is a crosssectional view of FIG. 3A taken along line B-B.

FIG. 4A is a perspective view schematically illustrating a configurationof the light source used in the planar lighting device of FIGS. 1 and 2;FIG. 4B is a cross sectional view of the light source illustrated inFIG. 4A; and FIG. 4C is a schematic perspective view illustrating oneLED of the light source of FIG. 4A as enlarged.

FIG. 5 is a schematic perspective view illustrating a shape of theinventive light guide plate.

FIG. 6 is a graph illustrating measurements representing a relationshipbetween Φ·N_(p)·L_(G)·K_(C) and light use efficiency.

FIG. 7 is a graph illustrating measurements representing illuminances oflight emitted by light guide plates each having different particledensities.

FIG. 8 is a graph illustrating relationships between light useefficiency and illuminance unevenness on the one hand and particledensity on the other.

FIG. 9 is a graph illustrating a distribution of illuminance measured ina direction parallel to the plane of incidence and passing through thecenter of the light guide plate.

FIG. 10 is a graph illustrating an array density of LED chips in adirection parallel to the plane of incidence and passing through thecenter of the light guide plate.

FIG. 11 is a graph illustrating a illuminance distribution obtainedusing the LED chips arranged at the array density of FIG. 10 as measuredin a direction parallel to the plane of incidence and passing throughthe center of the light guide plate.

FIG. 12 is a graph illustrating an illuminance distribution obtainedusing the LED chips arranged at the array density of FIG. 10 as measuredin a direction parallel to the auxiliary light entrance planes of thelight guide plate and passing through the center thereof.

FIG. 13 is a graph illustrating measurements of light use efficiencyobtained using light guide plates having different configurations.

DETAILED DESCRIPTION OF THE INVENTION

The planar lighting device of the invention will be described in detailbelow referring to an embodiment illustrated in the accompanyingdrawings.

FIG. 1 is a schematic perspective view illustrating a liquid crystaldisplay device provided with the planar lighting device of theinvention; FIG. 2 is a cross sectional view of the liquid crystaldisplay device illustrated in FIG. 1 taken along line II-II.

FIG. 3A is a view of the planar lighting device (also referred to as“backlight unit” below) illustrated in FIG. 2 taken along line III-III;FIG. 3B is a sectional view of FIG. 3A taken along line B-B.

A liquid crystal display device 10 comprises a backlight unit 20, aliquid crystal display panel 12 disposed on the side of the backlightunit closer to the light exit plane, and a drive unit 14 for driving theliquid crystal display panel 12. In FIG. 1, part of the liquid crystaldisplay panel 12 is not shown to better illustrate the configuration ofthe planar lighting device.

In the liquid crystal display panel 12, electric field is partiallyapplied to liquid crystal molecules, previously arranged in a givendirection, to change the orientation of the molecules. The resultantchanges in refractive index in the liquid crystal cells are used todisplay characters, figures, images, etc., on the liquid crystal displaypanel 12.

The drive unit 14 applies a voltage to transparent electrodes in theliquid crystal display panel 12 to change the orientation of the liquidcrystal molecules, thereby controlling the transmittance of the lighttransmitted through the liquid crystal display panel 12.

The backlight unit 20 is a lighting device for illuminating the wholesurface of the liquid crystal display panel 12 from behind the liquidcrystal display panel 12 and comprises a light exit plane havingsubstantially a same shape as an image display surface of the liquidcrystal display panel 12.

As illustrated in FIGS. 1, 2, 3A and 3B, the first example of thebacklight unit 20 comprises a main body of the lighting device 24 and ahousing 26. The main body of the lighting device 24 comprises two mainlight sources 28, two auxiliary light sources 29, a light guide plate30, and an optical member unit 32. The housing 26 comprises a lowerhousing 42, an upper housing 44, turnup members 46, and support members48. As illustrated in FIG. 1, a power supply casing 49 is provided onthe underside of the lower housing 42 of the housing 26 to hold powersupply units that supply the main light sources 28 and the auxiliarylight sources 29 with electrical power.

Now, components that make up the backlight unit 20 will be described.

The main body of the lighting device 24 comprises the main light sources28 and the auxiliary light sources 29 for emitting light, the lightguide plate 30 for admitting the light emitted by the main light sources28 and the auxiliary light sources 29 to produce planar light, and theoptical member unit 32 for scattering and diffusing the light producedby the light guide plate 30 to obtain light with further reducedunevenness.

First, the main light sources 28 and the auxiliary light sources 29 willbe described.

The main light sources 28 and the auxiliary light sources 29 basicallyhave the same configuration except the position with respect to thelight guide plate 30. Therefore, only the main light sources 28 will bedescribed as representative.

FIG. 4A is a perspective view schematically illustrating a configurationof the main light sources 28 of the planar lighting device 20 of FIGS. 1and 2; FIG. 4B is a cross sectional view of the main light source 28illustrated in FIG. 4A; and FIG. 4C is a schematic perspective viewillustrating only one LED of the main light source 28 of FIG. 4A asenlarged.

As illustrated in FIG. 4A, the main light source 28 comprises lightemitting diode chips (referred to as “LED chips” below) 50 and a lightsource mount 52.

The LED chip 50 is a chip of a light emitting diode emitting blue lightthe surface of which has a fluorescent substance applied thereon. It hasa light emission face 58 with a given area through which white light isemitted.

Specifically, when blue light emitted through the surface of lightemitting diode of the LED chip 50 is transmitted through the fluorescentsubstance, the fluorescent substance generates fluorescence. Thus, whenblue light emitted by the LED chip 50 is transmitted through thefluorescent substance, the blue light emitted by the light emittingdiode and the light radiated as the fluorescent substance generatesfluorescence blend to produce and emit white light.

The LED chip 50 may for example be formed by applying a YAG (yttriumaluminum garnet) base fluorescent substance to the surface of a GaN baselight emitting diode, an InGaN base light emitting diode, etc.

As illustrated in FIG. 4B, the light source mount 52 comprises an arraybase 54 and fins 56. The LED chips 50 described above are arranged in asingle row on the array base 54 at given intervals. Specifically, theLED chips 50 are arrayed along the length of a first light entranceplane 30 d or a second light entrance plane 30 e of a light guide plate30 to be described, that is, parallel to a line in which the first lightentrance plane 30 d or the second light entrance plane 30 e meets with alight exit plane 30 a.

The array base 54 is a plate member disposed such that one surfacethereof faces the thinnest lateral end face of the light guide plate 30,i.e., the first light entrance plane 30 d or the second light entranceplane 30 e of the light guide plate 30. The LED chips 50 are carried ona lateral plane of the array base 54 facing the light entrance plane 30b of the light guide plate 30.

The array base 54 according to the embodiment under discussion is formedof a metal having a good heat conductance as exemplified by copper andaluminum. The array base 54 also acts as a heat sink to absorb heatgenerated by the LED chips 50 and releases the heat to the outside.

The fins 56 are plate members each formed of a metal having a good heatconductance as exemplified by copper and aluminum. The fins 56 areconnected to the array base 54 on the side thereof opposite from the LEDchips 50 and spaced a given distance from neighboring fins 56.

A plurality of fins 56 provided in the light source mount 52 ensure alarge surface area and a high heat dissipation efficiency, increasingthe efficiency with which the LED chips 50 are cooled.

The heat sink may be not only of air-cooled type but also ofwater-cooled type.

While the embodiment under discussion uses the array base 54 of thelight source mount 52 as heat sink, a plate member without aheat-releasing function may be used to form the array base in place ofthe array base having a function of a heat sink, where the LED chipsneed not be cooled.

As illustrated in FIG. 4C, the LED chips 50 of the embodiment underdiscussion each have a rectangular shape such that the sidesperpendicular to the direction in which the LED chips 50 are arrayed areshorter than the sides lying in the direction in which the LED chips 50are arrayed or, in other words, the sides lying in the direction ofthickness of the light guide plate 30 to be described, i.e., thedirection perpendicular to the light exit plane 30 a, are the shortersides. Expressed otherwise, the LED chips 50 each have a shape definedby b>a where “a” denotes the length of the sides perpendicular to thelight exit plane 30 a of the light guide plate 30 and “b” denotes thelength of the sides in the array direction. Now, let “q” be the distanceby which the arrayed LED chips 50 are spaced apart from each other, thenq>b holds. Thus, the length “a” of the sides of the LED chips 50perpendicular to the light exit plane 30 a of the light guide plate 30,the length “b” of the sides in the array direction, and the distance “q”by which the arrayed LED chips 50 are spaced apart from each otherpreferably have a relationship satisfying q>b>a.

Providing the LED chips 50 each having the shape of a rectangle allows athinner design of the light source to be achieved while producing alarge amount of light. A thinner light source, in turn, enables athinner design of the planar lighting device to be achieved. Further,the number of LED chips that need to be arranged may be reduced.

While the LED chips 50 each preferably have a rectangular shape with theshorter sides lying in the direction of the thickness of the light guideplate 30 for a thinner design of the light source, the present inventionis not limited thereto, allowing the LED chips to have any shape asappropriate such as a square, a circle, a polygon, and an ellipse.

While the LED chips, arranged in a single row, has a monolayeredstructure in the embodiment under discussion, the present invention isnot limited thereto; one may use multilayered LED arrays for the lightsource comprising LED arrays each carrying LED chips 50 on the arraybase 54. Where the LEDs are thus stacked, more LED arrays can be stackedwhen the LED chips 50 are each adapted to have a rectangular shape andwhen the LED arrays are each adapted to have a reduced thickness. Wherethe LED arrays are stacked to form a multilayer structure, that is tosay, where more LED arrays (LED chips) are packed into a given space, alarge amount of light can be generated. Preferably, the above expressionalso applies to the distance separating the LED chips of an LED arrayfrom the LED chips of the LED arrays in adjacent layers. Expressedotherwise, the LED arrays preferably are stacked such that the LED chipsare spaced a given distance apart from the LED chips of the LED arraysin adjacent layers.

In the liquid crystal display device 10, provided with the main lightsources 28 and the auxiliary light sources 29 as in the example underdiscussion, the auxiliary light sources 29 are disposed opposite thefirst light entrance plane 30 f and the second light entrance plane 30 gof the light guide plate 30 thus providing four light entrance planes toadmit light also through the lateral sides of the light guide plate 30.Thus, the absolute value of the illuminance of light emitted through thelight exit plane 30 a can be improved and the amount of overall lightprovided by the liquid crystal display device 10 can be increased.

Further, the configuration where the light guide plate has the lightsources provided on all the four sides thereof allows a large amount ofillumination light to be emitted through the light exit plane, therebyachieving a larger display screen or area for devices in which the lightguide plate is used.

According to the invention, the amount of light emitted by the lightsources is preferably so adjusted that the illuminance distribution ofthe light emitted by the light guide plate represents a bell-curvedistribution. That is, a bell-curve distribution of illuminance suitablefor a display device such as a liquid crystal television is achieved bysetting a different amount of light for each of the LED chipsconstituting at least the main light sources.

In the main light sources 28 according to the embodiment underdiscussion, the LED chips 50 are independently set to their respectiveamounts of light such that the amount of light produced is greatestadjacent the central portion or area in the longitudinal direction ofthe first light entrance plane 30 d and the second light entrance plane30 e, decreasing with the increasing distance from the center towardboth ends.

The LED chips 50 are preferably set to their respective amounts of lightsuch that the illuminance of light as measured along the bisector α ofthe light guide plate 30 illustrated in FIG. 3A represents ahigh-in-the-middle, bell-curve distribution. Specifically, suppose thatthe amount of light of the LED chips opposite the central portion of thefirst light entrance plane 30 d and the second light entrance plane 30 eis 1, then the LED chips 50 are set to their respective amounts of lightsuch that an amount of light I satisfies 0<I≦1 in any other position.

According to the invention, the illuminance of the light emitted throughthe light exit plane 30 a can be formed into a bell-curve distribution,i.e., a distribution where the illuminance gradually increases towardthe center, by independently setting the amount of each LED chip 50 suchthat the LED chips disposed opposite the central portion of the lightentrance plane produce a greater amount of light than those disposedopposite the periphery. Where the light emitted through the light exitplane exhibits a bell-curve illuminance distribution so as to providethe central portion of the light entrance plane with the highestilluminance, difference in illuminance between the central area and theperiphery appears to be evened out by visual observation such thatuniform light seems to be emitted through the light exit plane. Thus,the planar lighting device of the invention is capable of emitting lightwith an illuminance distribution that is suitable for use in liquidcrystal televisions and the like.

To find such a desired illuminance distribution, one may for example usea calculation based on a sequential iteration method or any otherappropriate known method.

Preferably, in the second example described above, the LED chips 50provided on the auxiliary light sources 29 are also independently set totheir respective amounts of light as are those on the major lightsources 28. That is, where a different amount of light is setindependently for each of the LED chips constituting the main lightsources and the auxiliary light sources, a bell-curve brightnessdistribution suitable for a display device such as a liquid crystaltelevision can be achieved. Thus, different illuminances can be set indifferent areas as designated in the light exit plane 30 a. In otherwords, the amount of light can be set two-dimensionally in the lightexit plane 30 a. Accordingly, an area control is made possible wherebythe brightness in the light exit plane can be adjusted from area toarea.

The bell-curve illuminance distribution as measured on the light exitplane can also be achieved with substantially the same effects byadjusting the array density of the LED chips otherwise than by settingthe amount of light of the LED chips independently. Specifically, theLED chips 50 a and 50 b provided on the main light sources 28 may bearranged in a row at an array density that varies according to theirposition along the length of the first light entrance plane 30 d and thesecond light entrance plane 30 e opposite which these LED chips aredisposed to achieve a bell-curve brightness distribution.

Further, since the LED chips 50 are so arranged that the LED chips awayfrom the center of the main light source 28 in the array direction maybe reduced in number, the manufacturing costs can be reduced and so canpower consumption as well.

Now, the light guide plate 30 will be described.

FIG. 5 is a perspective view schematically illustrating theconfiguration of the light guide plate 30.

As illustrated in FIGS. 2, 3, and 5, the light guide plate 30 comprisesthe light exit plane 30 a, which is flat and substantially rectangular;two light entrance planes, the first light entrance plane 30 d and thesecond light entrance plane 30 e, formed on both sides of the light exitplane 30 a and substantially perpendicular to the light exit plane 30 a;two inclined planes, a first inclined plane 30 b and a second inclinedplane 30 c, located on the opposite side from the light exit plane 30 a,i.e., on the underside of the light guide plate so as to be symmetricalto each other with respect to a central axis, or the bisector αbisecting the light exit plane 30 a (see FIGS. 1 and 3) in a directionparallel to the first light entrance plane 30 d and the second lightentrance plane 30 e, and inclined a given angle with respect to thelight exit plane 30 a; and two auxiliary light entrance planes, a firstlight entrance plane 30 f and a second light entrance 30 g, formedsubstantially vertical to the light exit plane 30 a on the sides of thelight exit plane 30 a on which the light entrance planes are not formed,i.e., on the two sides perpendicular to the sides where the light exitplane 30 a and the light entrance planes meet.

The first inclined plane 30 b and the second inclined plane 30 c are soinclined as to be distanced farther from the light exit plane 30 a withthe increasing distance from the first light entrance plane 30 d and thesecond light entrance plane 30 e, respectively: expressed otherwise, thethickness of the light guide plate 30 in the direction perpendicular tothe light exit plane 30 a increases from the first light entrance plane30 d and the second light entrance plane 30 e toward the center of thelight guide plate 30.

Thus, the light guide plate 30 is thinnest at both sides thereof, i.e.,at the first light entrance plane 30 d and the second light entranceplane 30 e, and thickest at the center, i.e., on the bisector α, wherethe first inclined plane 30 b and the second inclined plane 30 c meet.Expressed otherwise, the light guide plate 30 has such a configurationthat the thickness of the light guide plate 30 in the directionperpendicular to the light exit plane 30 a increases with the increasingdistance from the first light entrance plane 30 d or the second lightentrance plane 30 e. The inclination angle of the first inclined plane30 b and the second inclined plane 30 c with respect to the light exitplane 30 a is not specifically limited.

The two main light sources 28 mentioned above are disposed opposite thefirst light entrance plane 30 d and the second light entrance plane 30 eof the light guide plate 30, respectively. Specifically, one of the mainlight sources 28 comprising LED chips 50 a and a light source mount 52 ais disposed opposite the first light entrance plane 30 d and the othermain light source 28 comprising LED chips 50 b and a light source mount52 b is disposed opposite the second light entrance plane 30 e. In theembodiment under discussion, the light emission face 58 of the LED chips50 of the light sources 28 has substantially the same length as thefirst light entrance plane 30 d and the second light entrance plane 30 ein the direction perpendicular to the light exit plane 30 a.

Thus, the planar lighting device 20 has the two major light sources 28disposed in such a manner as to sandwich the light guide plate 30. Inother words, the light guide plate 30 is placed between the two majorlight sources 28 arranged opposite each other with a given distancebetween them.

Further, the above-mentioned two auxiliary light sources 29 are eachdisposed opposite the first auxiliary light entrance plane 30 f and thesecond light entrance plane 30 g. Specifically, one of the auxiliarylight sources 29 configured by LED chips 50 c and a light source mount52 c is disposed opposite the first auxiliary light entrance plane 30 fand the other auxiliary light source 29 configured by LED chips 50 d anda light source mount 52 d is disposed opposite the second auxiliarylight entrance plane 30 g.

In the light guide plate 30 illustrated in FIG. 2, light entering thelight guide plate 30 through the first light entrance plane 30 d and thesecond light entrance plane 30 e is scattered as it travels through theinside of the light guide plate 30 by scatterers contained inside thelight guide plate 30 as will be described later in detail and, directlyor after being reflected by the first inclined plane 30 b or the secondinclined plane 30 c, exits through the light exit plane 30 a. Some lightcan in the process leak through the first inclined plane 30 b and thesecond inclined plane 30 c. However, it is then reflected by thereflection plate 34 provided on the side of the light guide plate closerto the first inclined plane 30 b and the second inclined plane 30 c toenter the light guide plate 30 again. The reflection plate 34 will bedescribed later in detail.

Likewise, the light emitted by the auxiliary light sources 29 andadmitted through the first auxiliary light entrance plane 30 f and thesecond auxiliary light entrance plane 30 g is scattered as it travelsthrough the inside of the light guide plate 30 by scatterers containedinside the light guide plate 30 as will be described later in detailand, directly or after being reflected by the first inclined plane 30 bor the second inclined plane 30 c, exits through the light exit plane 30a.

The shape of the light guide plate thus growing thicker in the directionperpendicular to the light exit plane 30 a with the increasing distancefrom the first light entrance plane 30 d or the second light entranceplane 30 e opposite which the main light source 28 is disposed allowsthe light admitted through the light entrance planes to travel fartherfrom the light entrance planes and, hence, enables a larger light exitplane to be achieved. Moreover, since the light entering through thelight entrance plane is advantageously guided to travel a long distancefrom the light entrance plane, a thinner design of the light guide plateis made possible.

The light guide plate 30 is formed of a transparent resin into whichscattering particles are kneaded and dispersed. Transparent resinmaterials that may be used to form the light guide plate 30 includeoptically transparent resins such as PET (polyethylene terephthalate),PP (polypropylene), PC (polycarbonate), PMMA (polymethyl methacrylate),benzyl methacrylate, MS resins, and COP (cycloolefin polymer). Thescattering particles kneaded and dispersed into the light guide plate 30may be formed, for example, of TOSPEARL (trademark), silicone, silica,zirconia, or a derivative polymer. The light guide plate 30 containingthe scattering particles is capable of emitting uniform illuminationlight through the light exit plane with a greatly reduced level ofbrightness unevenness. The light guide plate 30 so formed may bemanufactured using an extrusion molding method or an injection moldingmethod.

Now, let Φ be the scattering cross section of scattering particlescontained in the light guide plate 30; L_(G) the length in the incidentdirection from the first light entrance plane 30 d or the second lightentrance plane 30 e of the light guide plate 30 to a position where thethickness of the light guide plate 30 in the direction perpendicular tothe light exit plane 30 a is greatest, said incident direction,expressed otherwise, being the direction parallel to the direction inwhich light entering the light guide plate travels and perpendicular tothe line in which the light exit plane and the light entrance planes,i.e., the first light entrance plane and the second light entranceplane, meet, said length L_(G) being, in the embodiment underdiscussion, a half of the length of the light guide plate in theincident direction, which in the embodiment under discussion is thedirection perpendicular to the first light entrance plane 30 d of thelight guide plate 30, as also referred to as “direction of the opticalaxis” below, or, still otherwise expressed, the length from the firstlight entrance plane or the second light entrance plane to the bisectorα; N_(p) the density of the scattering particles contained in the lightguide plate 30, said density denoting the number of particles in unitvolume; and K_(C) a compensation coefficient. Then the valueΦ·N_(p)·L_(G)·K_(C) is preferably not less than 1.1 and not greater than8.2; the compensation coefficient K_(C) is preferably not less than0.005 and not greater than 0.1. The light guide plate 30, containingscattering particles satisfying the above relationship, is capable ofemitting uniform illumination light through the light exit plane 30 awith a greatly reduced level of brightness unevenness.

When parallel rays of light are caused to enter an isotropic medium, atransmittance T is generally expressed according to the Lambert-Beer lawby the following expression (1):T=I/I _(o)=exp(−ρ·x)  (1)

where x is a distance, I_(o) an intensity of incident light, I anintensity of outgoing light, and ρ an attenuation constant.

The above attenuation constant ρ can be expressed using the scatteringcross section of particles Φ and the number of particles N_(p) in unitvolume contained in the medium as follows:ρ=ΦN _(p)  (2)

Accordingly, the light extraction efficiency E_(out) is expressed by thefollowing expression (3) where L_(G) is the length of the light guideplate in the direction parallel to the direction in which light enteringthe light guide plate travels from the light entrance planes of thelight guide plate as far as the thickest position or, in the embodimentunder discussion, a half of the length of the light guide plate in thedirection of the optical axis. Said half of the length of the lightguide plate in the direction of the optical axis denoted by L_(G) is thelength of the light guide plate 30 in the direction perpendicular to thelight entrance planes of the light guide plate 30 from one of the lightentrance planes of the light guide plate 30 to the center of the lightguide plate 30.

The light extraction efficiency E_(out) is a ratio of light reaching theposition spaced apart from the light entrance plane of the light guideplate by the length L_(G) in the direction of the optical axis to theincident light. In the case of the light guide plate 30 illustrated inFIG. 2, for example, the light extraction efficiency E_(out) is a ratioof light reaching the center of the light guide plate or, lighttraveling half the length of the light guide plate in the direction ofthe optical axis to the light incident on either end plane.E_(out)∝exp(−Φ·N_(p)·L_(G))  (3)

The expression (3) applies to a space of limited dimensions. Introducingthe compensation coefficient K_(C) therein to correct the relationshipwith the expression (1), the light extraction efficiency E_(out) isexpressed by the following expression (4). The compensation coefficientK_(C) is a dimensionless compensation coefficient empirically obtainedwhere light propagates through an optical medium of limited dimensions.E _(out)=exp(−Φ·N _(p) ·L _(G) ·K _(C))  (4)

According to the expression (4), when Φ·N_(p)·L_(G)·K_(C) is 3.5, thelight extraction efficiency E_(out) is 3%. When Φ·N_(p)·L_(G)·K_(C) is4.7, the light extraction efficiency E_(out) is 1%.

The results show that the light extraction efficiency E_(out) decreasesas Φ·N_(p)·L_(G)·K_(C) increases. The light extraction efficiencyE_(out) decreases in such a manner presumably because light is scatteredincreasingly as it travels in the direction of the optical axis of thelight guide plate.

It follows, therefore, that the greater the value Φ·N_(p)·L_(G)·K_(C)is, the more preferable it is as a property for the light guide plate.When Φ·N_(p)·L_(G)·K_(C) is great, light exiting through a planeopposite the light entrance plane can be reduced whereas light emittedthrough the light exit plane can be increased. Expressed otherwise, whenΦ·N_(p)·L_(G)·K_(C) is great, the ratio of light emitted through thelight exit plane to the light incident on the light entrance planes canbe increased. That ratio is also referred to as “light use efficiency”below. Specifically, a light use efficiency as high as 50% or more isachieved when Φ·N_(p)·L_(G)·K_(C) is 1.1 or greater.

While light emitted through the light exit plane 30 a of the light guideplate 30 increasingly exhibits illuminance unevenness asΦ·N_(p)·L_(G)·K_(C) increases, the illuminance unevenness can be held tounder a given, tolerable level by holding the value Φ·N_(p)·L_(G)·K_(C)to 8.2 or less. Note that illuminance and brightness can be treatedsubstantially equally. Thus, it is assumed that brightness andilluminance possess similar tendencies in the present invention.

Thus, the value Φ·N_(p)·L_(G)·K_(C) of the inventive light guide plate30 is preferably not less than 1.1 and not greater than 8.2, and morepreferably not less than 2.0 and not greater than 8.0. Still morepreferably, the value Φ·N_(p)·L_(G)·K_(C) is not less than 3.0 and, mostpreferably, not less than 4.7.

The compensation coefficient K_(C) is preferably not less than 0.005 andnot greater than 0.1, thus 0.005≦K_(C)≦0.1.

Now, the light guide plate 30 will be described in greater detail byreferring to specific examples.

A computer simulation was conducted to obtain light use efficiencies fordifferent light guide plates given different values ofΦ·N_(p)·L_(G)·K_(C) by varying the scattering cross section Φ, theparticle density N_(p), the length L_(G), which is a half of the lengthof the light guide plate in the direction of the optical axis, and thecompensation coefficient K_(C). Further, illuminance unevenness wasevaluated. The illuminance unevenness (%) was defined as[(I_(Max)−I_(Min))/I_(Ave)]×100, where I_(Max) was a maximum illuminanceof light emitted through the light exit plane of the light guide plate,I_(Min) a minimum illuminance, and I_(Ave) an average illuminance.

Because the incoming light emitted by the auxiliary light sources has aconstant illuminance irrespective of the position in the optical axisdirection, the evaluation of the example was made under conditions wherethe auxiliary light sources are not provided.

The measurement results are shown in Table 1. In Table 1, judgments “O”indicate cases where the light use efficiency is 50% or more and theilluminance unevenness is 150% or less whereas judgments “X” indicatecases where the light use efficiency is less than 50% or the illuminanceunevenness is more than 150%.

TABLE 1 Light use Illuminance N_(p) L_(G) efficiency unevenness Φ [m²][pcs/m³] [m] K_(C) Φ · N_(p) · L_(G) · K_(C) [%] [%] Judgment Example 12.0 × 10⁻¹² 2.2 × 10¹⁴ 0.3 0.03 3.51 81.6 84 ◯ Example 2 2.0 × 10⁻¹² 4.3× 10¹⁴ 0.3 0.02 6.21 84.7 149 ◯ Example 3 2.0 × 10⁻¹² 8.6 × 10¹⁴ 0.10.02 3.86 82.8 82 ◯ Example 4 1.1 × 10⁻¹⁰ 1.5 × 10¹³ 0.3 0.008 3.91 83.0105 ◯ Example 5 1.1 × 10⁻¹⁰ 2.0 × 10¹³ 0.3 0.007 4.98 84.3 142 ◯ Example6 1.1 × 10⁻¹⁰ 3.5 × 10¹³ 0.1 0.007 2.86 79.2 47 ◯ Comparative 2.0 ×10⁻¹² 2.2 × 10¹³ 0.3 0.05 0.66 29.1 51 X example 1 Comparative 1.1 ×10⁻¹² 2.5 × 10¹² 0.3 0.01 0.99 43.4 59 X example 2 Comparative 4.8 ×10⁻¹⁸ 8.6 × 10¹⁷ 0.1 15.2 6.26 84.8 201 X example 3 Comparative 4.8 ×10⁻¹⁸ 1.7 × 10¹⁸ 0.1 13.9 11.5 84.9 225 X example 4

FIG. 6 illustrates a relationship between Φ·N_(p)·L_(G)·K_(C) and lightuse efficiency, i.e., the ratio of light emitted through the light exitplane 30 a to light incident on the light entrance planes.

Table 1 and FIG. 8 show that given Φ·N_(p)·L_(G)·K_(C) of 1.1 or more, ahigh light use efficiency, specifically 50% or more, is achieved whereasgiven Φ·N_(p)·L_(G)·K_(C) of 8.2 or less, illuminance unevenness can beheld to 150% or less.

It is also shown that given K_(c) of 0.005 or more, a high light useefficiency is achieved, and given K_(c) of 0.1 or less, illuminanceunevenness observed in light emitted from the light guide plate can bereduced to a low level.

Next, light guide plates varying in particle density N_(p) of theparticles kneaded or dispersed therein were fabricated to measurebrightness distributions of light emitted at different positions in thelight exit plane of the individual light guide plates. In the embodimentunder discussion, the conditions including scattering cross section Φ,length L_(G), which is a half of the length of the light guide plate inthe direction of its optical axis, compensation coefficient K_(C), andshape of the light guide plate, but excluding particle density N_(p),were respectively set to fixed values as the measurements were made. Inthe embodiment under discussion, therefore, the valueΦ·N_(p)·L_(G)·K_(C) changes in proportion as the particle density N_(p)changes.

FIG. 7 shows the measurements of the distribution of illuminanceobserved in the light emitted through the light exit plane of theindividual light guide plates having different particle densities. FIG.7 shows the illuminance [lx] on the vertical axis plotted against alight guiding length, which is the distance [mm] from one of the lightentrance planes of the light guide plate on the horizontal axis.

Illuminance unevenness was calculated from[(I_(Max)−I_(Min))/I_(Ave)]×100[%], where I_(Max) is a maximumilluminance in the measured distribution of light emitted from areas ofthe light exit plane close to the lateral ends thereof, I_(Min) is aminimum illuminance, and I_(Ave) is an average illuminance.

FIG. 8 illustrates a relationship between the calculated illuminanceunevenness and particle density. FIG. 8 shows the illuminance unevenness[%] on the vertical axis plotted against the particle density[pieces/m³] on the horizontal axis. Also shown in FIG. 10 is arelationship between light use efficiency and particle density, theparticle density being likewise indicated on the horizontal axis and thelight use efficiency [%] on the vertical axis.

As shown in FIGS. 7 and 8, increasing the particle density or,consequently, increasing Φ·N_(p)·L_(G)·K_(C), results in an enhancedlight use efficiency but then illuminance unevenness also increases. Thegraphs also show that reducing the particle density or, consequently,reducing Φ·N_(p)·L_(G)·K_(C), results in lowered light use efficiencybut then illuminance unevenness decreases.

Φ·N_(p)·L_(G)·K_(C) of not less than 1.1 and not greater than 8.2 yieldsa light use efficiency of 50% or more and illuminance unevenness of 150%or less. Illuminance unevenness, when reduced to 150% or less, isinconspicuous.

Thus, it will be understood that Φ·N_(p)·L_(G)·K_(C) of not less than1.1 and not greater than 8.2 yields light use efficiency above a certainlevel and a reduced illuminance unevenness.

According to the light guide plate 30 as described above used in theexample under discussion, the absolute value of the illuminance improvesby a factor of about 1.5 where the main light sources 28 and theauxiliary light sources 29 are provided as compared with a case whereonly the main light sources 28 are provided.

Next, the optical member unit 32 will be described.

The optical member unit 32 serves to reduce the brightness unevenness ofthe illumination light emitted through the light exit plane 30 a of thelight guide plate 30 to achieve emission of light with reducedbrightness unevenness through a light emission plane 24 a of the mainbody of the lighting device 24. As illustrated in FIG. 2, the opticalmember unit 32 comprises a diffusion sheet 32 a for diffusing theillumination light emitted through the light exit plane 30 a of thelight guide plate 30 to reduce brightness unevenness, a prism sheet 32 bhaving micro prism arrays formed parallel to the lines where the lightexit plane and the light entrance planes meet, and a diffusion sheet 32c for diffusing the illumination light emitted through the prism sheet32 b to reduce brightness unevenness.

The diffusion sheets 32 a and 32 c and the prism sheet 32 b may beprovided by making use, for example, of the diffusion sheets and theprism sheets disclosed in paragraphs [0028] through [0033] of JP2005-234397 A by the Assignee or the Applicant of the presentapplication.

While the optical member unit in the embodiment under discussioncomprises the two diffusion sheets 32 a and 32 c and the prism sheet 32b between the two diffusion sheets, there is no specific limitation tothe order in which the prism sheet and the diffusion sheets are arrangedor the number thereof to be provided. Nor are the prism sheet and thediffusion sheets specifically limited, and use may be made of variousoptical members, provided that they are capable of reducing thebrightness unevenness of the illumination light emitted through thelight exit plane 30 a of the light guide plate 30.

For example, the optical members may also be formed of transmittanceadjusting members each comprising a number of transmittance adjustersconsisting of diffusion reflectors distributed according to thebrightness unevenness in addition to or in place of the diffusion sheetsand the prism sheet described above. Further, the optical member unitmay be adapted to have two layers formed using one sheet each of theprism sheet and the diffusion sheet or two diffusion sheets only.

Now, the reflection plate 34 of the main body of the lighting devicewill be described.

The reflection plate 34 is provided to reflect light leaking through thefirst inclined plane 30 b and the second inclined plane 30 c of thelight guide plate 30 back into the light guide plate 30 and helpsenhance the light use efficiency. The reflection plate 34 is shapedaccording to the contour of the first inclined plane 30 b and the secondinclined plane 30 c of the light guide plate 30 to cover the firstinclined plane 30 b and the second inclined plane 30 c. In theembodiment under discussion, the reflection plate 34 is shaped tocontour the sectionally triangular shape formed by the first inclinedplane 30 b and the second inclined plane 30 c as illustrated in FIG. 2.

The reflection plate 34 may be formed of any material as desired,provided that it is capable of reflecting light leaking through theinclined planes of the light guide plate 30. The reflection plate 34 maybe formed, for example, of a resin sheet produced by kneading, forexample, PET or PP (polypropylene) with a filler and then drawing theresultant mixture to form voids therein for increased reflectance; asheet with a specular surface formed by, for example, depositingaluminum vapor on the surface of a transparent or white resin sheet; ametal foil such as an aluminum foil or a resin sheet carrying a metalfoil; or a thin sheet metal having a sufficient reflective property onthe surface.

Upper light guide reflection plates 36 are disposed between the lightguide plate 30 and the diffusion sheet 32 a, i.e., on the side of thelight guide plate 30 closer to the light exit plane 30 a, covering themain light sources 28 and the end portions of the light exit plane 30 a,i.e., the end portion thereof closer to the first light entrance plane30 d and the end portion thereof closer to the second light entranceplane 30 e. Thus, the upper light guide reflection plates 36 aredisposed to cover an area extending from part of the light exit plane 30a of the light guide plate 30 as far as part of the array bases 54 ofthe main light sources 28 in a direction parallel to the direction ofthe optical axis. Briefly, two upper light guide reflection plates 36are disposed respectively on both end portions of the light guide plate30.

The upper light guide reflection plates 36 thus provided prevents lightemitted by the main light sources 28 from leaking toward the light exitplane 30 a instead of entering the light guide plate 30.

Thus, light emitted from the LED chips 50 of the main light sources 28is efficiently admitted through the first light entrance plane 30 d andthe second light entrance plane 30 e of the light guide plate 30,increasing the light use efficiency.

The lower light guide reflection plates 38 are disposed on the side ofthe light guide plate 30 opposite from the light exit plane 30 a, i.e.,on the same side as the first inclined plane 30 b and the secondinclined plane 30 c, covering part of the main light sources 28. Theends of the lower light guide reflection plates 38 closer to the centerof the light guide plate 30 are connected to the reflection plate 34.

The upper light guide reflection plates 36 and the lower light guidereflection plates 38 may be formed of any of the above-mentionedmaterials used to form the reflection plate 34.

The lower light guide reflection plates 38 prevent light emitted by thelight sources 28 from leaking toward the first inclined plane 30 b andthe second inclined plane 30 c of the light guide plate 30 instead ofentering the light guide plate 30.

Thus, light emitted from the LED chips 50 of the main light sources 28is efficiently admitted through the first light entrance plane 30 d andthe second light entrance plane 30 e of the light guide plate 30,increasing the light use efficiency.

While the reflection plate 34 is connected to the lower light guidereflection plates 38 in the embodiment under discussion, theirconfiguration is not so limited; they may be formed of separatematerials.

The shapes and the widths of the upper light guide reflection plates 36and the lower light guide reflection plates 38 are not limitedspecifically, provided that light emitted by the main light sources 28is reflected and directed toward the first light entrance plane 30 d orthe second light entrance plane 30 e such that light emitted by the mainlight sources 28 can be admitted through the first light entrance plane30 d or the second light entrance plane 30 e and then guided toward thecenter of the light guide plate 30.

While, in the embodiment under discussion, the upper light guidereflection plates 36 are disposed between the light guide plate 30 andthe diffusion sheet 32 a, the location of the upper light guidereflection plates 36 is not so limited; it may be disposed between thesheets constituting the optical member unit 32 or between the opticalmember unit 32 and the upper housing 44.

Further, the upper light guide reflection plates 36 and the lower lightguide reflection plates 38 are preferably provided also at the ends ofthe first auxiliary light entrance plane 30 f and the second lightentrance plane 30 g of the light guide plate 30. Where the upper lightguide reflection plates 36 and the lower light guide reflection plates38 are provided also at the ends of the first auxiliary light entranceplane 30 f and the second light entrance plane 30 g of the light guideplate 30, the light emitted by the auxiliary light sources 29 can beefficiently admitted into the light guide plate.

Next, the housing 26 will be described.

As illustrated in FIG. 2, the housing 26 accommodates and securestherein the main body of the lighting device 24 by holding it from aboveand both sides thereof, i.e., the light emission plane 24 a and thefirst inclined plane 30 b and the second inclined plane 30 c. Thehousing 26 comprises the lower housing 42, the upper housing 44, theturnup members 46, and the support members 48.

The lower housing 42 is open at the top and has a configurationcomprising a bottom section and lateral sections provided upright on thefour sides of the bottom section. Briefly, it has substantially theshape of a rectangular box open on one side. As illustrated in FIG. 2,the bottom section and the lateral sections support the main body of thelighting device 24 placed therein from above on the underside and on thelateral sides and covers the faces of the main body of the lightingdevice 24 except the light emission plane 24 a, i.e., the plane oppositefrom the light emission plane 24 a of the main body of the lightingdevice 24 (rear side) and the lateral sections.

The upper housing 44 has the shape of a rectangular box; it has anopening at the top smaller than the rectangular light emission plane 24a of the main body of the lighting device 24 and is open on the bottomside.

As illustrated in FIG. 2, the upper housing 44 is placed from above themain body of the lighting device 24 and the lower housing 42, that is,from the light exit plane side, to cover the main body of the lightingdevice 24 and the lower housing 42, which holds the former, as well asfour lateral sections 22 b.

The turnup members 46 have a substantially U-shaped sectional profilethat is identical throughout their length. That is, each turnup member46 is a bar-shaped member having a U-shaped profile in cross sectionperpendicular to the direction in which it extends.

As illustrated in FIG. 2, the turnup members 46 are fitted between thelateral sections of the lower housing 42 and the lateral sections of theupper housing 44 such that the outer face of one of the parallelsections of said U shape connects with lateral sections 22 b of thelower housing 42 whereas the outer face of the other parallel sectionconnects with the lateral sections of the upper housing 44.

To connect the lower housing 42 with the turnup members 46 and theturnup members 46 with the upper housing 44, one may use any knownmethod such as a method using bolts and nuts and a method using bonds.

Thus providing the turnup members 46 between the lower housing 42 andthe upper housing 44 increases the rigidity of the housing 26 andprevents the light guide plate from warping. As a result, for example,light can be efficiently emitted without, or with a greatly reducedlevel of, brightness unevenness. Further, even where the light guideplate used is liable to develop a warp, the warp can be corrected withan increased certainty or the warping of the light guide plate can beprevented with an increased certainty, thereby allowing light to beemitted through the light exit plane without brightness unevenness orwith a greatly reduced level of brightness unevenness.

While the upper housing, the lower housing and the turnup members of thehousing may be formed of various materials including metals and resins,lightweight, high-rigidity materials are preferable.

While the turnup members are discretely provided in the embodiment underdiscussion, they may be integrated with the upper housing or the lowerhousing. Alternatively, the configuration may be formed without theturnup members.

The support members 48 have an identical profile in cross sectionperpendicular to the direction in which they extend throughout theirlength. That is, each support member 48 is a bar-shaped member having anidentical cross section perpendicular to the direction in which itextends.

As illustrated in FIG. 2, the support members 48 are provided betweenthe reflection plate 34 and the lower housing 42, more specifically,between the reflection plate 34 and the lower housing 42 close to theend of the first inclined plane 30 b of the light guide plate 30 onwhich the first light entrance plane 30 d is located and close to theend of the second inclined plane 30 c of the light guide plate 30 onwhich the second light entrance plane 30 e is provided. The supportmembers 48 thus secure the light guide plate 30 and the reflection plate34 to the lower housing 42 and support them.

With the support members 48 supporting the reflection plate 34, thelight guide plate 30 and the reflection plate 34 can be brought into aclose contact. Furthermore, the light guide plate 30 and the reflectionplate 34 can be secured to a given position of the lower housing 42.

While the support members are discretely provided in the embodimentunder discussion, the invention is not limited thereto; they may beintegrated with the lower housing 42 or the reflection plate 34. To bemore specific, the lower housing 42 may be adapted to have projectionsto serve as support members or the reflection plates may be adapted tohave projections to serve as support members.

The locations of the support members are also not limited specificallyand they may be located anywhere between the reflection plate and thelower housing. To stably hold the light guide plate, the support membersare preferably located closer to the ends of the light guide plate or,in the embodiment under discussion, near the first light entrance plane30 d and the second light entrance plane 30 e.

The support members 48 may be given various shapes and formed of variousmaterials without specific limitations. For example, two or more of thesupport members may be provided at given intervals.

Further, the support members may have such a shape as to fill the spaceformed by the reflection plate and the lower housing. Specifically, thesupport members may have a shape such that the side thereof facing thereflection plate has a contour following the surface of the reflectionplate and the side thereof facing the lower housing has a contourfollowing the surface of the lower housing. Where the support membersare adapted to support the whole surface of the reflection plates,separation of the light guide plate and the reflection plate can bepositively prevented and, further, generation of brightness unevennessthat might otherwise be caused by light reflected by the reflectionplates can be prevented.

The planar lighting device 20 is basically configured as describedabove.

In the light guide plate 30, light emitted by the main light sources 28provided on both sides of the light guide plate 30 strikes the lightentrance planes, i.e., the first light entrance plane 30 d and thesecond light entrance plane 30 e, of the light guide plate 30 whilelight emitted by the auxiliary light sources 29 provided on the othertwo sides of the light guide plate 30 strikes the auxiliary lightentrance planes, i.e., the first auxiliary light entrance plane 30 f andthe second auxiliary light entrance plane 30 g. Then, the light admittedthrough the respective planes is scattered by scatterers containedinside the light guide plate 30 as will be described later in detail asthe light travels through the inside of the light guide plate 30 and,directly or after being reflected by the first inclined plane 30 b orthe second inclined plane 30 c, exits through the light exit plane 30 a.In the process, part of the light leaking through the first inclinedplane 30 b and the second inclined plane 30 c is reflected by thereflection plate 34 to enter the light guide plate 30 again.

Thus, light emitted through the light exit plane 30 a of the light guideplate 30 is transmitted through the optical member 32 and emittedthrough the light emission plane 24 a of the main body of the lightingdevice 24 to illuminate the liquid crystal display panel 12.

The liquid crystal display panel 12 uses the drive unit 14 to controlthe transmittance of the light according to the position so as todisplay characters, figures, images, etc. on its surface.

The illuminance distribution of light emitted through the light exitplane was measured using the inventive planar lighting device.

In the example used for the measurement, the light guide plate had ashape as defined by the following dimensions: the length from the firstauxiliary light entrance plane 30 f to the second auxiliary lightentrance plane 30 g measured 1000 mm; a length “d” of the first lightentrance plane 30 d and the second light entrance plane 30 e in thedirection perpendicular to the light exit plane measured 580 mm; thelength from the light exit plane 30 a to the rear side at the bisectorα, or, a maximum thickness D, measured 3.5 mm; and a length L_(G) fromthe first light entrance plane 30 d or the second light entrance plane30 e to the bisector α measured 290 mm.

The weight ratio of the scattering particles mixed into the light guideplate to the light guide plate was 0.07 Wt %.

FIG. 9 illustrates illuminance distribution of light emitted through thelight exit plane of the planar lighting device provided with the lightguide plate 30 having the above configuration where the illuminancedistribution illustrated was measured along the middle of the lightguide plate parallel to the first light entrance plane 30 d and thesecond light entrance plane 30 e, i.e., on the bisector α.

FIG. 9 indicates relative illuminance [lx] on the vertical axis plottedagainst the position [mm] in the longitudinal direction of the firstlight entrance plane 30 d and the second light entrance plane 30 e ofthe light guide plate 30 given on the horizontal axis. The position “0”on the horizontal axis indicates the center of the light guide plate 30in the longitudinal direction of the first light entrance plane 30 d andthe second light entrance plane 30 e; the positions “−500” and “500”indicate both ends of the light guide plate 30 in the longitudinaldirection of the first light entrance plane 30 d and the second lightentrance plane 30 e.

FIG. 10 illustrates an array density of the LED chips 50 varyingaccording to the position in the longitudinal direction of the firstlight entrance plane 30 d and the second light entrance plane 30 e.

FIG. 10 is a graph of the LED chip distribution illustrating the arraydensity of the LED chips 50 a and 50 b along the length of the mainlight sources 28 of the inventive planar lighting device 10.

FIG. 10 indicates on the horizontal axis the positions [sections] on thelight guide plate 30 each having a given unit length, which is 4 mm inthe example under discussion, into which the longitudinal length of thefirst light guide plate 30 d and the second light guide plate 30 e isdivided, plotted against the number of LED chips [pieces] arranged ineach section having a given unit length on the vertical axis. Thesection “0” on the horizontal axis indicates the 4-mm range containingthe center of the light guide plate 30 in the longitudinal direction ofthe first light entrance plane 30 d and the second light entrance plane30 e; the sections “−12” and “12” indicate the outermost ranges betweeneach of the outermost ends of the light guide plate 30 and a point 4 mmfrom said end toward the center in the longitudinal direction of thefirst light entrance plane 30 d and the second light entrance plane 30e.

The example under discussion uses two different array density patterns,I1 and X1, to array the LED chips 50 as illustrated in FIG. 10.

The array density is preferably so determined as to provide ahigh-in-the-middle, bell-curve illuminance distribution as measuredalong a bisector a of the light guide plate 30. To find an array densityfor the LED chips 50 whereby a bell-curve illuminance distribution isobtained, one may for example use a calculation based on a sequentialiteration method or any other appropriate known method.

In the example under discussion, the LED chips 50 are arranged at adensity of 14 pieces per section (4-mm range) in the array densitypatterns I1 and X1 in the positions opposite the central portion of thefirst light entrance plane 30 d and the second light entrance plane 30 eof the light guide plate 30, i.e., the position “0” on the horizontalaxis, as illustrated in FIG. 10. The array density of the LED chips 50decreases with the increasing distance from the center of the firstlight entrance plane 30 d and the second light entrance plane 30 e suchthat the number of LED chips 50 per section is 0 in the positions “−11”and “11” in the case of array density pattern I1 and in the positions“−12” and “12” in the case of array density pattern X1, i.e., thepositions opposite both ends of the first light entrance plane 30 d andthe second light entrance plane 30 e.

In both patterns I1 and X1, the array density peaks adjacent the centralportion of the first light entrance plane 30 d and the second lightentrance plane 30 e, i.e., adjacent the position “0” on the horizontalaxis in FIG. 10. Now, let the array density be 1 at the center, then theLED chips 50 are arrayed at an array density D satisfying 0<D≦1 in anyother position.

The LED chips 50 are preferably arranged such that the array density ishighest at a central portion in the lengthwise direction of the firstlight entrance plane 30 d and the second light entrance plane 30 e, thearray density decreasing with the increasing distance from the center.Thus, a bell-curve distribution can be obtained for the light emitted bythe light guide plate 30.

FIGS. 11 and 12 illustrate illuminance distributions obtained using thelight guide plate 30 where the LED chips are arrayed at the densitygiven in FIG. 10.

FIG. 11 illustrates illuminance distributions of the light as measuredon the light exit plane 30 a in a direction parallel to the longitudinaldirection of the first light entrance plane 30 d and the second lightentrance plane 30 e and passing through the center of the light guideplate 30. In this case, the LED chips 50 were arranged at the arraydensity patterns I1 and X1 of FIG. 10. FIG. 11 indicates the relativeilluminance [lx] on the vertical axis plotted against the position [mm]in the longitudinal direction of the first light entrance plane 30 d andthe second light entrance plane 30 e of the light guide plate 30 on thehorizontal axis.

As illustrated in FIG. 11, whether the array density pattern I1 or X1 isused, the illuminance peaks adjacent the central area, i.e., adjacentthe position “0” on the horizontal axis and decreases down theperiphery, representing a bell-curve illuminance distribution.

FIG. 12 illustrates an illuminance distribution of light on the lightexit plane 30 a in a direction parallel to the longitudinal direction ofthe first auxiliary light entrance plane 30 f and the second auxiliarylight entrance plane 30 g and passing through the center of the lightguide plate 30. Note that in the example under discussion, the arraydensity of the LED chips 50 is constant throughout the length of theauxiliary light sources 29.

FIG. 12 indicates the relative illuminance [lx] on the vertical axisplotted against the position [mm] in the longitudinal direction of thefirst auxiliary light entrance plane 30 f and the second auxiliary lightentrance plane 30 g on the horizontal axis.

As is apparent from FIG. 12, the illuminance distribution along thelength of the first auxiliary light entrance plane 30 f and the secondauxiliary light entrance plane 30 g represents a bell curve distributionwhere the illuminance peaks at the center, i.e., adjacent the position“0” on the horizontal axis, decreasing with the increasing distance fromthe center toward both ends, almost regardless of whether the LED chips50 on the main light sources 28 are arrayed at the array density patternI1 or X1.

It follows, therefore, that where the LED chips 50 are provided on themain light sources 28 with such array densities as illustrated in FIG.10, illuminance distributions as illustrated in FIGS. 11 and 12 can beobtained where the illuminance adjacent the central portion of the lightexit plane 30 a is higher than in the periphery thereof, representing ahigh-in-the-middle, bell-curve illuminance distribution.

Further, in the embodiment under discussion, let D1 be the thickness ofthe light guide plate at its light entrance plane (thickness of thelight guide plate at a location at which light is admitted); D2 thethickness of the light guide plate at a location where the light guideplate is thickest, which, in the embodiment under discussion, is thethickness of the light guide plate where the bisector α thereof islocated (thickness at the center); and L the length of the light guideplate in the incident direction from the first light entrance plane tothe second light entrance plane (light guiding length), L being 2L_(G)in the embodiment under discussion. Then, it is preferable that thefollowing relationships hold:D1<D2 and27/100000<(D2−D1)/(L/2)<5/100  (A); andthat the ratio Npa of the weight of the scattering particles containedto the weight of the light guide plate satisfies a range:0.04 Wt %<Npa<0.25 Wt %.When the above relationships are satisfied, the light emissionefficiency of the main light sources can be increased to 30% or more.

Alternatively, it is also preferable that the light guide plate isimproved such that the following relationships hold:D1<D2 and66/100000<(D2−D1)/(L/2)<26/1000  (B); andthat the ratio Npa of the weight of the contained scattering particlesto the weight of the light guide plate satisfies a range:0.04 Wt %<Npa<0.25 Wt %.When the above relationships are satisfied, the light emissionefficiency of the main light sources can be increased to 40% or more.

It is preferable that the light guide plate is improved such that thefollowing relationships hold:D1<D2 and1/1000<(D2−D1)/(L/2)<26/1000  (C); andthat the ratio Npa of the weight of the contained scattering particlesto the weight of the light guide plate satisfies a range of:0.04 Wt %<Npa<0.25 Wt %.When the above relationships are satisfied, the light emissionefficiency can be increased to 50% or more.

FIG. 13 illustrates measurements of the light use efficiency of lightguide plates of which the inclined planes have different inclinationangles from each other, i.e., light guide plates having various shapeswith different values of (D2−D1)/(L/2). Because the light guide platesaccording to the example used for the measurements have a flat shape inthe direction in which light emitted by the auxiliary light sourcestravels and, therefore, light use efficiency is substantially notchanged by the shape of the inclined planes, only the main light sourceswere provided to measure the light use efficiency thereof.

FIG. 13 indicates (D2−D1)/(L/2) of the light guide plate on thehorizontal axis plotted against light use efficiency [%] on the verticalaxis.

As will be apparent from the measurements illustrated in FIG. 13, whenthe light guide plate has a shape satisfying27/100000<(D2−D1)/(L/2)<5/100, the light use efficiency can be increasedto 30% or more; when the light guide plate has a shape satisfying66/100000<(D2−D1)/(L/2)<26/1000, the light use efficiency can beincreased to 40% or more; and when the light guide plate has a shapesatisfying 1/1000<(D2−D1)/(L/2)<26/1000, the light use efficiency can beincreased to 50% or more.

The light guide plate may have any shape without limitation to the aboveshape, provided that the thickness of the light guide plate increaseswith the increasing distance from the light entrance plane.

For example, prism arrays may be formed on the first inclined plane 30 band the second inclined plane 30 c in the direction parallel to thefirst light entrance plane 30 d and the second light entrance plane 30e. Instead of such prism arrays, optical elements similar to prisms maybe provided and arranged regularly. For example, elements having lenseffects such as lenticular lenses, concave lenses, convex lenses, oroptical elements in pyramidal shape may be formed on the inclined planesof the light guide plate.

In the embodiment under discussion, the light exit plane 30 a of thelight guide plate 30 has the longer sides adjacent the light entranceplanes and the shorter sides adjacent the auxiliary light entranceplanes in order to emit light through the light exit plane with anenhanced brightness and efficiency. The invention, however, is notlimited to such a configuration; the light exit plane may be formed intoa square or the sides thereof adjacent the light entrance planes may bethe shorter sides and the sides thereof adjacent the auxiliary lightentrance planes may be the longer sides.

The joint between the first inclined plane and the second inclined planeof the light guide plate preferably has a rounded shape. Where the jointbetween the first inclined plane and the second inclined plane of thelight guide plate has a rounded shape connecting the two planessmoothly, generation of brightness unevenness observed as, for example,a bright line on the line of intersection between the first inclinedplane and the second inclined plane can be prevented.

While, in the embodiment under discussion, the inclined planes of thelight guide plate are defined by a straight line in cross section, theshape of the first inclined plane and the second inclined plane, i.e.,the underside of the light guide plate, is not limited specifically. Thefirst inclined plane and the second inclined plane may be defined by acurved surface or each of them may have two or more inclined sections.In other words, each inclined plane may have inclined sections havingdifferent inclination angles according to their position. Further, theinclined planes may be curved outwardly or inwardly with respect to thelight exit plane, or may have outwardly and inwardly curved sectionscombined.

The inclined planes preferably have a configuration such that theirinclination angle with respect to the light exit plane decreases fromthe light entrance planes toward the center of the light guide plate ortoward a position where the light guide plate is thickest. Where theinclination angle of the inclined planes gradually decreases, lighthaving less brightness unevenness can be emitted through the light exitplane.

The inclined planes more preferably have an aspherical cross sectionthat may be expressed by a 10-th order polynomial. Where the inclinedplanes have such a configuration, light having less brightnessunevenness can be emitted regardless of the thickness of the light guideplate.

Further, the shape of the light guide plate is not limited to that ofthe embodiment under discussion; for example, the first inclined planeand the second inclined plane may have different inclinations from eachother. Still further, the light guide plate may have a shape such thatthe distance between the first light entrance plane and the positionwhere the light guide plate is thickest is different from the distancebetween the second light entrance plane and the position where the lightguide plate is thickest. Described otherwise, the length of the firstinclined plane in the direction of the optical axis is different fromthat of the second inclined plane in the direction of the optical axis.

The light guide plate having such a shape is also capable of allowinglight to travel a long distance from the light entrance planes, whilekeeping a thin design. This enables a reduced thickness of the lightguide plate and a larger light exit plane to be achieved.

Again, with the light guide plate having the above shape, preferably thevalue Φ·N_(p)·L_(G)·K_(C) is in the range of not less than 1.1 and notgreater than 8.2, and 0.005≦K_(C)≦0.1, where L_(G) is the length in theincident direction from the light entrance plane to the position wherethe thickness of the light guide plate in the direction perpendicular tothe light exit plane is thickest. When the above ranges are satisfied,light can be emitted through the light exit plane with a reducedilluminance unevenness and a high light use efficiency.

The light guide plate may be fabricated by mixing a plasticizer into atransparent resin.

Fabricating the light guide plate from a material thus prepared bymixing a transparent material and a plasticizer provides a flexiblelight guide plate, allowing the light guide plate to be deformed intovarious shapes. Accordingly, the surface of the light guide plate can beformed into various curved surfaces.

Where the light guide plate is given such flexibility, the light guideplate or the planar lighting device using the light guide plate can evenbe mounted to a wall having a curvature when used, for example, for adisplay board employing ornamental lighting (illuminations).Accordingly, the light guide plate can be used for a wider variety ofapplications and in a wider application range including ornamentallighting and POP (point-of-purchase) advertising.

Said plasticizer is exemplified by phthalic acid esters, or,specifically, dimethyl phthalate, diethyl phthalate (DEP), dibutylphthalate (DBP), di(2-ethylhexyl) phthalate (DOP (DEHP)), di-n-octylphthalate (DnOP), diisononyl phthalate (DINP), dinonyl phthalate (DNP),diisodecylphthalate (DIDP), phthalate mixed-base ester (C₆ to C₁₁)(610P, 711P, etc.) and butylbenzyl phthalate (BBP). Besides phthalicacid esters, said plasticizer is also exemplified by dioctyl adipate(DOA), diisononyl adipate (DINA), dinormal alkyl adipate (C_(6, 8, 10))(610A), dialkyl adipate (C_(7, 9)) (79A), dioctyl azelate (DOZ), dibutylsebacate (DBS), dioctyl sebacate (DOS), tricresyl phosphate (TCP),tributyl acetylcitrate (ATBC), epoxidized soybean oil (ESBO), trioctyltrimellitate (TOTM), polyesters, and chlorinated paraffins.

While the inventive planar lighting device has been described above indetail, the invention is not limited in any manner to the aboveembodiment and various improvements and modifications may be madewithout departing from the spirit of the present invention.

For example, while each LED chip of the light sources is formed byapplying YAG fluorescent substance to the light emission face of a blueLED, the LED chip may be formed otherwise without limitations to such aconfiguration. For example, the LED chip used herein may be formed usinga different monochromatic LED such as a red LED or a green LED with afluorescent substance.

Further, an LED unit formed using three kinds of LEDs, i.e., a red LED,a green LED, and a blue LED, may be used. In that case, light beamsemitted by the three kinds of LEDs are blended to produce white light.

Alternatively, a semiconductor laser (LD) may be used instead of an LED.

Further, one may provide between the light guide plate and each of thelight sources (main light sources and/or auxiliary light sources) aportion formed of a material having a refractive index close to that ofthe light guide plate. Alternatively, part of the light entrance planesand/or the auxiliary light entrance planes of the light guide plate maybe formed of a material having a smaller refractive index than the otherparts.

Where the part through which light emitted by the light source isadmitted is adapted to have a smaller refractive index than the otherparts, light emitted by the light source can be admitted moreefficiently, and the light use efficiency can be further enhanced.

Further, two or more light guide plates may be juxtaposed by connectingtheir auxiliary light entrance planes to provide a single light exitplane formed by a plurality of light guide plates. In that case, theauxiliary light sources may be provided only on the auxiliary lightentrance planes of the outermost light guide plates.

In the embodiment under discussion, the auxiliary light sources areprovided opposite the auxiliary light entrance planes of the light guideplate in order to further increase the brightness of the light emittedthrough the light exit plane and obtain a bell-curve brightnessdistribution as observed in the two mutually orthogonal directions.However, the invention is not limited to such a configuration; one mayuse a configuration, for example, whereby the auxiliary light sourcesare not provided and only the main light sources are provided to admitlight through the light entrance planes.

1. A planar lighting device comprising: a light guide plate including: alight exit plane being flat and rectangular, for emitting planar light;four light entrance planes, each for admitting light traveling parallelto said light exit plane, and each being formed along each of four sidesof said light exit plane; and a pair of rear planes being formed on aside opposite from said light exit plane and inclined such that athickness of said light guide plate in a direction perpendicular to saidlight exit plane grows thicker with an increasing distance from each ofa pair of opposite light entrance planes in said four light entranceplanes and joining each other in a middle between said pair of oppositelight entrance planes; and four light sources, each for emitting each ofsaid four light entrance planes and for admitting the emitted lightthrough said four light entrance planes, and each being disposedopposite each of said four light entrance planes of said light guideplate, wherein each of said four light sources has LED chips and asupport for supporting said LED chips, and wherein said LED chips ofeach of said four light sources are arrayed on a plane of said supportfacing each of said four light entrance planes in a longitudinaldirection of each of said four light entrance planes.
 2. The planarlighting device according to claim 1, wherein said LED chips compriseLEDs emitting the light having different amounts of light, respectively.3. The planar lighting device according to claim 1, wherein said LEDchips are adjusted independently of each other to their respectiveamounts of light.
 4. The planar lighting device according to claim 2,wherein an amount of light I of each of said LED chips is adjusted tosatisfy 0<I≦1 when an amount of light of one of said LED chips adjacenta central portion of each light entrance plane is
 1. 5. The planarlighting device according to claim 1, wherein said four light entranceplanes comprises: a first light entrance plane and a second lightentrance plane being formed on a pair of opposite sides of said lightexit plane and constituting said pair of opposite light entrance planes,and a third light entrance plane and a fourth light entrance plane beingformed on another pair of opposite sides of said light exit plane andconstituting another pair of opposite light entrance planes, whereinsaid pair of rear planes are composed of a pair of inclined planes thatare parallel to said light exit plane in a direction along said firstlight entrance plane and said second light entrance plane, respectively,come closest to said light exit plane at said first light entrance planeand said second light entrance plane, respectively, and are distancedfarthest from said light exit plane along a central joint of said pairof rear planes joining each other in the middle between said first lightentrance plane and said second light entrance plane.
 6. The planarlighting device according to claim 1, wherein each of said LED chips hasa configuration satisfying an inequality b>a where “a” denotes a lengthin a thickness direction of said light guide plate and “b” denotes alength in a direction perpendicular to said thickness direction of saidlight guide plate.
 7. The planar lighting device according to claim 1,wherein said light guide plate contains therein numerous scatteringparticles, and satisfies following inequalities:27/100000<(D2−D1)/(L/2)<26/1000 and0.04 Wt %<N_(p)<0.25 Wt % where N_(p) denotes a density of saidscattering particles, L denotes a distance between said pair of oppositelight entrance planes, D1 denotes a thickness at one of said pair ofopposite light entrance planes, and D2 denotes a thickness at a centerof said light guide plate.
 8. The planar lighting device according toclaim 1, wherein said light guide plate contains therein numerousscattering particles and satisfies following inequalities:1.1≦Φ·N _(p) ·L _(G) ·K _(C)≦8.20.005≦K_(C)≦0.1 where Φ denotes a scattering cross section of thescattering particles, N_(p) denotes a density of said scatteringparticles, K_(C) denotes a compensation coefficient, and L_(G) denotes alength in an incident direction of the light from one of said pair ofopposite light entrance planes to a position where said thickness ofsaid light guide plate in the direction perpendicular to said light exitplane is thickest.